This invention relates to commercial and industrial ink jet printers of the type commonly used for marking of products. Such devices require high speed and high reliability and must operate in somewhat hostile environments in terms of temperature, service intervals and the like. When an ink jet printer is being readied for use, it has been necessary to calibrate the printer for the particular characteristics of the ink and nozzle it is to use. In the prior art, it is known to use certain characteristics of the printing operation to approximate the nozzle drive voltage at which good printing operations can be obtained. For example, it is known to determine the infinite satellite condition and the foldback point of an ink drop stream. The former is a condition in which the small satellites which form between drops, neither forwardly nor rearwardly merge with the main drops. The foldback condition is an upper bound in which the breakoff point of the drops, relative to the nozzle, first reverses. The foldback condition is described more thoroughly in U.S. Pat. No. 5,196,860.
U.S. Pat. No. 5,196,860 to Pickell et al., assigned to the present assignee, detects one or both of these points and then selects a predetermined nozzle drive voltage somewhere between the two bounds. It does not, however, directly determine the true bounds of the print window. It relies on factory data concerning the ink and the nozzle to calculate a voltage that is expected to lie within the print window.
Another prior art method of estimating the drive voltage point within a print window is disclosed in U.S. Pat. No. 4,878,064 to Katerberg et al. In this patent, a D.C. voltage is applied to a drop charging electrode. The stream current is monitored as a function of charging voltage, and when a dip in the detected current appears this indicates that the satellites have been deflected by the deflection electrode. Further operation yields the foldback point from which a nozzle drive voltage is calculated as a fraction of the voltage at the foldback point. Again, this is an estimating technique dependant on factory data and detecting the foldback point.
The present invention, by contrast, is a method of automatically determining the actual nozzle drive "print window", that is, the range of nozzle drive voltages that provide substantially constant deflection of drops (i.e., desirable print quality) for a particular nozzle, ink type, font and stimulation voltage waveform combination. Thus, rather than estimating the print window, the nozzle operation is sampled and used to accurately determine the print window. It is possible therefore, for the first time, to test a printer whenever necessary, for example, upon installing a new nozzle or a different ink, thereby to positively determine the print window or range of nozzle drive values at which the printer can be operated to obtain good printing results.